The X-Ray Tube

DMI 50B

Kyle Thornton

Evolution Of The X-Ray Tube

Roentgen experimented with a Crookes tube– Evacuated tube containing two electrodes– Could not control the number of electrons moving between the

electrons (tube current) In 1913, W.D. Coolidge redesigned the tube, calling it a

hot cathode tube– Replaced the negative electrode with a small spiral-wound

tungsten wire– Current was applied to this wire creating heat to free electrons

Hot cathode tubes became the standard for x-ray tubes

Cold Cathode X-Ray TubeEarly 20th century

Cold Cathode Tube Fluorescence

Coolidge Tube w/Stationary Anode

Helpful website

http://www.ndt-ed.org/EducationResources/HighSchool/Radiography/xraygenerators.htm

http://www.youtube.com/watch?v=Bc0eOjWkxpU

Common Components Of The X-Ray Tube

Glass envelope Cathode Anode

Glass Envelope

A vacuum is created prior to sealing the tube

The glass envelope: – Provides support for the

electrodes– Provides electrical insulation– Assists in the removal of heat– Is made of very thick glass to

withstand heat– Is thinner at the exit window

where the x-rays emerge from the tube

Cathode

Negative electrode of the x-ray tube

Consists of a filament and a focusing cup

The filament provides a source of electrons

Electrons are freed when the filament is heated

The filament is a long thin tungsten wire shaped into a spiral coil

Cathode

About 10 volts and 3 – 5 amperes are applied to the filament to heat it

Tungsten is used because of its high melting point - 3370° C

It is also malleable Most modern tubes contain two filaments

– The longer filament is used when large numbers of electrons are needed

– The shorter filament is used when lower tube currents and maximum detail are needed

The Focusing Cup

The filaments are mounted within a focusing cup Generally made of nickel, stainless steel, or

molybdenum A negative charge is placed on the focusing cup Focuses the electrons on a smaller spot of the anode This improves detail on the film

Benefit of the Focusing Cup

X-Ray Tube Focusing Cup

http://www.youtube.com/watch?v=LyWuvSZRSLc&feature=related

The Anode

The anode is the positive electrode

It is struck by the electron stream The area struck is the anode

target Usually made of tungsten A great deal of heat is produced

at the anode– Depends on the voltage, the

current, and the length of time the anode is struck by electrons

Stationary Anodes

Used when lower heat quantities are produced Consists of a tungsten target and copper block and

stem Copper conducts the heat away from the tungsten

target The heat can damage the anode target, causing pitting

– Results in x-ray beam of reduced intensity due to scattering and absorption in the uneven surface

Rotating Anodes

The first was made by Philips Medical Systems in Holland in 1929

Consists of a tungsten alloy disk on a molybdenum base

Rotating anodes range in size from about 5 cm to 12.5 cm

Disk sizes determine the thermal load Anodes have an angle of about 7° to 20°

Rotating Anodes

Rotating anodes assure that the same area of the target being struck over and over is rare

Heat energy is distributed more evenly over the anode face

More rapid exposures are possible The rotating anode is driven by an induction

motor

Purposes Of The Anode

Serves as a positive electrode Provides structural support for the target plate Provides a means for of dissipating heat to the

target

The Benefits Of Using Tungsten As A Target Material

Malleability High melting point -

3370° C High Z number Resists vaporization at

high temperatures Ability to conduct heat

away from area of heat production

Its density Ability to absorb heat

without raising the temperature of the conductor

Its availability makes it cost-effective

The Induction Motor

Works on the principle of Lenz’ law Using opposing magnetic fields, the copper bar is

made to rotate Rotation of the anode is accomplished more efficiently

this way The anode is surrounded by electromagnets that are

switched on and off in rapid sequence Anodes utilize 60 Hz AC Anodes usually rotate at 3600 rpm

– Newer anodes may rotate at 10,000 rpm

Induction Motor Diagram

Target Plate Angulation

The area of the target struck by the electron stream is the focal spot

Image sharpness is improved when the focal spot is small

By angling the target, a small area can be struck, but at the same time provides a large space for heat dissipation

Line-Focus Principle

By angling the target, the effective area is much smaller than the actual area of electron interaction

As the target angle decreases, so does the effective focal spot

Dual Focus Anodes

Heel Effect

A consequence of the line focus principle

The beam intensity on the cathode side is more intense than on the anode side

Patients must be positioned accordingly for some exams

Heel Effect

Off-Focus Radiation

Off-Focus Radiation

Radiation produced when electrons bounce off the target area of the anode and strike other areas and produce x-rays

About 15% of the electrons do this

This reduces image contrast Can be controlled by applying

an additional diaphragm close to the focal spot

Tube Exit Window

The useful beam is emitted from the tube exit window

This section of glass is generally much thinner than the rest of the tube

The Tube Housing

Supports and houses the x-ray tube Provides insulation Prevents shock Is lined with a lead tube shield to prevent

leakage radiation Oil surrounds the tube within the tube shielding

The Tube Housing

Causes Of X-Ray Tube Failure

A single excessive exposure Long exposure times Filament vaporization

– The most common

Safe Operation With Tube Rating Charts

Radiographic rating chart Anode cooling chart Housing cooling chart

Tube Rating Charts

Here’s Some Questions

Which of the following exposures are unsafe?– 95 kVp, 150 mA, 1 sec. 3400 rpm, .6mm focal spot– 80kVp, 400 mA, .5 sec. 3400 rpm, 1mm focal spot– 125 kVp, 500 mA, .1 sec 10,000 rpm 1 mm– 75 kVp, 700 mA, .3 sec, 10,000 rpm 1 mm focal

spot– 88 kVp, 400 mA, .1 sec., 10,000 rpm .6mm focal

spot

Heat Units

The product of mA, T, and kVp

HU = mA X T X kVp Used to determine

thermal capacity of an anode or tube housing

The tube housing cooling chart is similar to that of the anode cooling chart

Anode Cooling Chart

Here’s Some More Questions

If 50,000 HU are delivered to the anode, how long will it take to cool completely?

How many heat units are produced if 6 films are exposed using a three phase, six pulse generator at 82 kVp and 120 mAs